1. Field of the Invention
This invention relates to a silicon-system thin film, photovoltaic device using the same, method for forming the silicon-system thin film, and method for producing the photovoltaic device. The photovoltaic device of the present invention is applicable to various purposes, e.g., solar cells, sensors and imaging cells.
2. Related Background Art
Silicon-system thin films showing crystallinity have been formed by the cast method or the like which grows the film from the liquid phase. However, these methods need treatment of the films at high temperature, which tends to decrease productivity and increase cost.
In an attempt to solve these problems, Meier et al. propose a new method for producing solar cells at lower temperature, in which a microcrystalline solar cell of p-i-n structure is formed on a substrate kept at 220.degree. C. by glow discharge at a high frequency of 110 MHz ("ON THE WAY TOWARDS HIGH EFFICIENCY THIN FILM SILICON SOLAR CELLS BY THE "MICROMORPH" CONCEPT," J. Meier et al., Mat. Res. Soc. Symp. Proc., Vol. 420, p3, 1996). It is reported that the cell shows a photoelectric conversion efficiency of 7.7%. It is also reported that a stacked solar cell of amorphous and microcrystalline silicone films shows an efficiency of 13.1%.
The above method using glow discharge, although giving a microcrystalline silicon film of high photoelectric conversion efficiency, needs uneconomically much time to secure a necessary film thickness, because of insufficient film-making rate.
It is known that carrier mobility in a photovoltaic device that uses a silicon-system thin film showing crystallinity generally tends to be deteriorated by various causes, including silicon dangling bonds in the grain boundaries, strains evolving in the vicinity of the grain boundaries and imperfectness of the crystal itself, with the result that photoelectric conversion efficiency of the cell may not be sufficiently high.
One of the measures to reduce the above adverse effects is to increase crystallinity of the cell, which, however, involves decreased film-making rate, and additional steps, such as heat treatment of the substrate with electron beams, laser beams or heat from a lamp, or cycles of forming a silicon-system thin film and annealing in a hydrogen atmosphere. These should increase film-making time and production cost.
It is an object of the present invention to provide a silicon-system thin film and photovoltaic device using the same of high photoelectric conversion efficiency, capable of being produced at a commercially practical film-making rate by solving the above problems.